1. Field of the Invention
This invention relates to an image processing apparatus for reproducing half-tones, for example, a color-image processing apparatus having a color correcting function, and an image processing method implemented by these apparatus.
2. Description of the Prior Art
A copying apparatus equipped with a laser-beam printer is well known as an apparatus of the above-mentioned kind. The operation performed within the laser-beam printer involves pulse-width modulating an image signal by means of a reference signal (e.g., a triangular-wave signal) and reproducing tones by making the obtained pulse width correspond to laser emission time.
The use of full-color images has recently increased not only in the printing and design fields but also in the office, and this has been accompanied by the popularization of color copying apparatus which faithfully read and output color images. Here also it is well-known to employ the aforementioned pulse-width modulated laser-beam printer in order to faithfully reproduce color tones.
FIG. 1 is a simplified view showing the construction of an image reading section. Shown in FIG. 1 are an original 1, a platen 2 on which the original 1 is placed, a lamp 3 for illuminating the original 1, an imaging element array 4, an infrared cutting filter 5, a contact-type CCD sensor (hereinafter referred to simply as a xe2x80x9cCCDxe2x80x9d) 6, and an optical unit 7 which holds the lamp 3, imaging element array 4, filter 5 and CCD 6.
The above-described arrangement is so adapted that when a copy key (not shown) is pressed, the lamp 3 illuminates the original 1 and light reflected from the original 1 is imaged on the CCD 6 through the imaging element array 4 and infrared cutting filter 5.
The optical unit 7 is scanned in the direction of the arrow, and electrical signals of main and subordinate scans are produced by the CCD 6 and sent to a signal processing circuit shown in FIGS. 2A and 2B.
FIGS. 2A and 2B are block diagrams illustrating the construction of an image forming apparatus. A signal from the CCD 6 is amplified by an amplifier 8, and the amplified signal is converted into digital image data by an A/D converting circuit 9. The resulting data is converted into image (density) data by a LOG converter circuit 29, the data is then subjected to various types of image processing by an image processing circuit 30, and the processed digital data is fed into a D/A converter 14, where the digital signal is converted back into an analog signal. The analog signal is applied to a comparator 16, where the signal is pulse-width modulated by being compared with a signal of a predetermined period generated by a triangular-wave generating circuit 15. This pulse-width modulated binarized image signal is fed into a laser driving circuit 17 (FIG. 2B) in order to be used as a signal for on/off control of an emission from a laser diode 18. The laser light emitted by the laser diode 18 is made to scan in the main scanning direction by a well-known polygon mirror 19 an f/xcex8 lens 20 and a reflecting mirror 21 so as to irradiate a photosensitive drum 22, which serves as an image carrier rotating in the direction of the arrow, An electrostatic latent image is thus formed on the drum 22.
After the photosensitive drum 22 is uniformly de-electrified by an exposing device 28, it is uniformly electrified to a minus charge by a corona charging device 23. Thereafter, the drum 22 is acted upon by the laser light so that an electrostatic latent image conforming to the image signal is formed on its surface. A so-called image scanning method, in which the portions (black pixels) that undergo development are exposed, is one type of a developing method often used by a laser-beam printer. By means of a well-known reverse developing method, a developing device 24 causes a toner having a negative electrification characteristic to adhere to the portion of the photosensitive drum 22 de-electrified by the laser, thereby forming an image.
The developed image (the toner image having a negative charge) formed on the photosensitive drum 22 by the above-described method is transferred to a transfer material (generally paper) 26 by a transfer corona charging device 25. The visible toner image transferred to the transfer material 26 is then fixed by a fixing device (not shown). Residual toner left on the photosensitive drum is subsequently scraped off by a cleaner 27, after which the process described above is repeated.
In the image forming apparatus described above, conventionally the LOG converter circuit 29 performs a digital conversion by a look-up table method. For example, two types of look-up tables are provided and a changeover is made between a character mode and a photograph mode. An inconvenience encountered in the above-described prior art is that the changeover of the LOG conversion tables for deciding output density must be performed manually by an operator, who makes the changeover upon looking at the original. In the case of an original that is a photograph, maximum density differs greatly depending upon whether the original is a silver chloride photograph or printed matter.
Even silver chloride photographs are of various types, such as photographs that are bright overall and photographs that are dark and have almost no high density. Selecting the mode in each of these instances is difficult.
Furthermore, in a case where the appropriate LOG conversion table has been selected, the following problem can arise:
When a bright photographic original having many portions of a reflective density of 1.7-2.0 or greater is processed by a table of LOG 1.7 (in which reproduction is possible from a density of 0.05 to 1.7), tone reproduction of the portions having a density greater than 1.7 is impossible. Conversely, when LOG 1.7 is selected in case of a faint original whose maximum density is on the order of 1.0, the maximum copy density falls below 1.0 and an image having an overall faint density results.
In a case where a copied image is used as an original to produce another copy which is then copied in the same manner, the foregoing problem becomes more pronounced with each generation of such copies and picture quality deteriorates markedly.
Though a variety of color-image forming apparatus are known, those in widest use employ a laser printer in which image exposure on a photosensitive body is performed by laser light and the image is developed to obtain a visible image. Such a laser printer exhibits the advantages of high picture quality and speed and is widely employed as the output unit of a color copier or as an ordinary printer.
A color-image forming apparatus will now be described with reference to FIGS. 1, 3 and 4.
First, the process for reading a color original will be described. When a copy key (not shown) is pressed, the lamp 3 (FIG. 1) illuminates the original 1 and light reflected from the original 1 is imaged on the CCD 6 through the imaging element array 4 and infrared cutting filter 5, thereby forming an image of the original on the CCD 6. The optical unit 7 is made to scan the original 1 successively in the direction of the arrow. As shown in FIG. 3, filters for red (R), green (G) and blue (B) are correctly mounted on the CCD 6 for each and every pixel. As the original is scanned, electrical signals from the CCD 6 are processed by a signal processing circuit shown in FIG. 4.
In FIG. 4, 6B, 6G, 6R represent signals from each of the B, G, R elements on thee CCD 6 shown in FIG. 3. The R, G, B signals are applied to an A/D converter circuit 41 and a LOG converter circuit 42, whereby these signals are converted into signals Y1, M1, C1 of digital form. The signals Y1, M1, C1 are introduced to a circuit 43, which performs black extraction and UCR (under-color rejection) to produce signals Y2, M2, C2, Bk2 by processing expressed by the following equations:
Y2=Y1xe2x88x92k3(Y1, M1, C1)min
M2=M1xe2x88x92k3(Y1, M1, C1)min
C2=C1xe2x88x92k3(Y1, M1, C1)min
Bk2=k1(Y1, M1, C1)min+k2
Here (Y1, M1, C1)min represents the minimum of each of the signals Y1, M1, C1, and k1, k2, k3 represent coefficients. The Y2, M2, C2 signals are introduced to a color separating filter of the CCD 6 and to a color corrector circuit 44 in order to correct the spectral distribution of the colorants used in in a printer. The color correcting circuit 44 performs the processing indicated by the following equation:       [                                        Y            3                                                            M            3                                                            C            3                                ]    =            [                                                  a              11                                                          a              12                                                          a              13                                                                          a              21                                                          a              22                                                          a              23                                                                          a              31                                                          a              32                                                          a              33                                          ]        ⁡          [                                                  Y              2                                                                          M              2                                                                          C              2                                          ]      
In the above equation, a11-a33 represent masking coefficients for color correction. The coefficients k1-k3 and a11-a33 are stored in a ROM (a read-only memory). When copying has started, these coefficients are set in the circuits 43 and 44 by a CPU (central processing unit).
Signals Y3, M3, C3, Bk3, which are the result of processing by the circuit 44, are developed by color-image forming means (printer 45), such as a color thermal-transfer printer, color ink-jet printer or color laser printer. When a laser printer is used as the means for developing the image, the image is formed by so-called pulse-width modulation, namely modulation in which the emission time of the laser is made to correspond to the size of the image signal.
The above-mentioned masking coefficients of the color correcting means are decided by a processing procedure such as described below.
First, a standard color original whose chromaticity coordinates are evident is read by the optical unit 7 shown in FIG. 1, and the masking coefficients are decided by the method of least squares, shown below, in such a manner that the Y3, M3, C3, Bk3 signals from the color corrector circuit 14 illustrated in FIG. 4 are placed in the above-mentioned chromaticity coordinates.
Let Yxe2x80x2, Mxe2x80x2, Cxe2x80x2 (Y2, M2, C2 of FIG. 4) represent the signals for a standard color original of one color, and let Y, M, C represent chromaticity components serving as target values. In such case, respective color offsets xcex94Y, xcex94M, xcex94C from the target values are as expressed by the following equations:
xcex94Y=a11Yxe2x80x2+a12Mxe2x80x2+a13Cxe2x80x2xe2x88x92Y
xcex94M=a21Yxe2x80x2+a22Mxe2x80x2+a23Cxe2x80x2xe2x88x92M
xe2x80x83xcex94C=a31Yxe2x80x2+a32Mxe2x80x2+a33Cxe2x80x2xe2x88x92M
This is carried out for standard color originals of 20 colors of the kind shown in FIGS. 5A, 5B, 5C, by way of example, and the method of least squares is applied so as to minimize the offsets.
Let xcfx86Y, xcfx86M and xcfx86C represent the evaluation functions of yellow, magenta and cyan, respectively. For the color yellow, xcfx86Y is the sum of the squares of xcex94Y with regard to 20 colors. This is expressed as follows:       Φ    ⁢          xe2x80x83        ⁢    Y    =                    ∑                  i          =          1                20            ⁢              xe2x80x83            ⁢              Δ        ⁢                  xe2x80x83                ⁢                  Y          i          2                      =                  ∑                  i          =          1                20            ⁢              xe2x80x83            ⁢                        (                                                    a                11                            ⁢                              Y                i                xe2x80x2                                      +                                          a                12                            ⁢                              M                i                xe2x80x2                                      +                                          a                13                            ⁢                              C                i                xe2x80x2                                      -                          Y              i                                )                2            
where Yixe2x80x2, Mixe2x80x2 and Cixe2x80x2 represent signals for an i-th standard color original, and Yi represents the target value of yellow from among the color components of the i-th standard color original.
Since the coefficients are determined in such a manner that the partial differential of the above-mentioned evaluation functions will be zero, the following results:                                                         ∂              Φ                        ⁢                          xe2x80x83                        ⁢            Y                                ∂                          a              11                                      =                  xe2x80x83                ⁢                  2          ⁢                      {                                                            (                                      Σ                    ⁢                                          xe2x80x83                                        ⁢                                          Y                      i                      xe2x80x2                                        ⁢                                          Y                      i                      xe2x80x2                                                        )                                ⁢                                  a                  11                                            +                                                (                                      Σ                    ⁢                                          xe2x80x83                                        ⁢                                          M                      i                      xe2x80x2                                        ⁢                                          Y                      i                      xe2x80x2                                                        )                                ⁢                                  a                  12                                            +                                                (                                      Σ                    ⁢                                          xe2x80x83                                        ⁢                                          C                      i                      xe2x80x2                                        ⁢                                          Y                      i                      xe2x80x2                                                        )                                ⁢                                  a                  13                                            -                                                                                    xe2x80x83                    ⁢                      (                          Σ              ⁢                              xe2x80x83                            ⁢                              Y                i                            ⁢                              Y                i                xe2x80x2                                      )                    }                =        0                                                                    ∂              Φ                        ⁢                          xe2x80x83                        ⁢            Y                                ∂                          a              12                                      =                  xe2x80x83                ⁢                  2          ⁢                      {                                                            (                                      Σ                    ⁢                                          xe2x80x83                                        ⁢                                          Y                      i                      xe2x80x2                                        ⁢                                          M                      i                      xe2x80x2                                                        )                                ⁢                                  a                  11                                            +                                                (                                      Σ                    ⁢                                          xe2x80x83                                        ⁢                                          M                      i                      xe2x80x2                                        ⁢                                          M                      i                      xe2x80x2                                                        )                                ⁢                                  a                  12                                            +                                                (                                      Σ                    ⁢                                          xe2x80x83                                        ⁢                                          C                      i                      xe2x80x2                                        ⁢                                          M                      i                      xe2x80x2                                                        )                                ⁢                                  a                  13                                            -                                                                                    xe2x80x83                    ⁢                      (                          Σ              ⁢                              xe2x80x83                            ⁢                              Y                i                            ⁢                              M                i                xe2x80x2                                      )                    }                =        0                                                                    ∂              Φ                        ⁢                          xe2x80x83                        ⁢            Y                                ∂                          a              13                                      =                  xe2x80x83                ⁢                  2          ⁢                      {                                                            (                                      Σ                    ⁢                                          xe2x80x83                                        ⁢                                          Y                      i                      xe2x80x2                                        ⁢                                          C                      i                      xe2x80x2                                                        )                                ⁢                                  a                  11                                            +                                                (                                      Σ                    ⁢                                          xe2x80x83                                        ⁢                                          M                      i                      xe2x80x2                                        ⁢                                          C                      i                      xe2x80x2                                                        )                                ⁢                                  a                  12                                            +                                                (                                      Σ                    ⁢                                          xe2x80x83                                        ⁢                                          C                      i                      xe2x80x2                                        ⁢                                          C                      i                      xe2x80x2                                                        )                                ⁢                                  a                  13                                            -                                                                                    xe2x80x83                    ⁢                      (                          Σ              ⁢                              xe2x80x83                            ⁢                              Y                i                            ⁢                              C                i                xe2x80x2                                      )                    }                =        0            
Similarly, the coefficients are decided in such a manner that the partial differential for the evaluation functions of magenta and cyan will also be zero. As a result of the foregoing, the following equations are obtained:             [                                                  Σ              ⁢                              xe2x80x83                            ⁢                              Y                i                xe2x80x2                            ⁢                              Y                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              M                i                xe2x80x2                            ⁢                              Y                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              C                i                xe2x80x2                            ⁢                              Y                i                xe2x80x2                                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              M                i                xe2x80x2                            ⁢                              M                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              M                i                xe2x80x2                            ⁢                              M                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              C                i                xe2x80x2                            ⁢                              M                i                xe2x80x2                                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              Y                i                xe2x80x2                            ⁢                              C                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              M                i                xe2x80x2                            ⁢                              C                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              C                i                xe2x80x2                            ⁢                              C                i                xe2x80x2                                                        ]        ⁡          [                                                  a              11                                                          a              12                                                          a              13                                                                          a              21                                                          a              22                                                          a              23                                                                          a              31                                                          a              32                                                          a              33                                          ]        =      "AutoLeftMatch"          [                                                  Σ              ⁢                              xe2x80x83                            ⁢                              Y                i                xe2x80x2                            ⁢                              Y                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              M                i                xe2x80x2                            ⁢                              Y                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              C                i                xe2x80x2                            ⁢                              Y                i                xe2x80x2                                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              M                i                xe2x80x2                            ⁢                              M                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              M                i                xe2x80x2                            ⁢                              M                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              C                i                xe2x80x2                            ⁢                              M                i                xe2x80x2                                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              Y                i                xe2x80x2                            ⁢                              C                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              M                i                xe2x80x2                            ⁢                              C                i                xe2x80x2                                                                        Σ              ⁢                              xe2x80x83                            ⁢                              C                i                xe2x80x2                            ⁢                              C                i                xe2x80x2                                                        ]      
Expressing the above equation in the form [C] [A]=[D], results in [A]=[Cxe2x88x921][D], and the coefficients a11-a33 are found. It should be noted that [Cxe2x88x921] is the inverse matrix of [C].
The masking coefficients thus decided are stored beforehand in a ROM and a color reproduction of the original is made upon applying a color correction using these masking coefficients.
Color reproduction as performed in the example of the prior art described above has the following drawbacks:
In a case where the masking coefficients are decided by the method of least squares, the following is obtained with regard to the yellow evaluation function, by way of example:                               Φ          ⁢                      xe2x80x83                    ⁢          Y                =                  xe2x80x83                ⁢                              ∑                          i              =              1                        20                    ⁢                      xe2x80x83                    ⁢                      Δ            Yi            2                                                  =                  xe2x80x83                ⁢                              Δ            Y1            2                    +                      Δ            Y2            2                    +          …          +                      Δ            Y20            2                                                  =                  xe2x80x83                ⁢                                            (                                                                    a                    11                                    ⁢                                      Y                    1                    xe2x80x2                                                  +                                                      a                    12                                    ⁢                                      M                    1                    xe2x80x2                                                  +                                                      a                    13                                    ⁢                                      C                    1                    xe2x80x2                                                  -                                  Y                  1                                            )                        2                    +                                                  xe2x80x83                ⁢                                            (                                                                    a                    11                                    ⁢                                      Y                    2                    xe2x80x2                                                  +                                                      a                    12                                    ⁢                                      M                    2                    xe2x80x2                                                  +                                                      a                    13                                    ⁢                                      C                    2                    xe2x80x2                                                  -                                  Y                  2                                            )                        2                    +          …          +                                                  xe2x80x83                ⁢                              (                                                            a                  11                                ⁢                                  Y                  20                  xe2x80x2                                            +                                                a                  12                                ⁢                                  M                  20                  xe2x80x2                                            +                                                a                  13                                ⁢                                  C                  20                  xe2x80x2                                            -                              Y                20                                      )                    2                    
The first term in the above equation is the error in chromaticity in a case where the first color in FIG. 5A is reproduced, and the last term is the error in chromaticity in a case where the 20th color in FIG. 5C is reproduced. Since squared errors are added at the same weighting with regard to the chromaticity errors of the 1st through 20th colors, the masking coefficients that are finally created are such that the average of the reproducibility errors of each of the colors is minimized. Accordingly, when color reproduction is carried out using these masking coefficients, each color of the original always acquires a certain error from the chromaticity and is reproduced in the manner shown in FIG. 6. Consequently, when an original having many colors inclining to a specific color, say, an original having many colors of green family is reproduced, reproduction is always performed with the aforementioned error and, depending upon the image, an image exhibiting good color reproduction cannot always be obtained.
Image processing using a histogram of an original has been proposed in U.S. Pat. No. 4,667,228, U.S. Pat. No. 4,931,864, U.S. Ser. No. 775,013, U.S. Ser. No. 839,717, U.S. Pat. No. 4,907,286, U.S. Pat. No. 4,837,450, U.S. Pat. No. 4,903,145, and U.S. Ser. No. 592,334. Especially U.S. Pat. No. 4,667,228 and U.S. Pat. No. 4,931,864 disclose gamma correction for each of the color components based upon a histogram of an original.
Yet U.S. Pat. No. 4,667,228 and U.S. Pat. No. 4,931,864 each have a drawback that color balance as a whole is unbalanced, as correction coefficients are set for each of the color components. These prior art patents also have a drawback in that the dynamic range of input signals cannot be fully used, since the signals to be corrected are produced by an arithmetic operation using a plurality of color component signals.
Accordingly, an object of the present invention is to provide an image processing apparatus capable of achieving tone reproduction suited to the image.
According to the present invention, the foregoing object is attained by providing an image processing apparatus for converting luminance information based upon an original image into density information and outputting a signal for image formation, comprising generating means for generating luminance information of the original image, and creating means for creating a frequency distribution based upon the luminance information generated by the generating means, wherein processing coefficients of the conversion which convert luminance information into density information are controlled, depending upon the distribution state of the frequency distribution.
Another object of the present invention is to provide a color-image forming apparatus capable of improving color reproducibility of widely distributed colors in the image of an original, thereby making it possible to obtain an image having excellent shades.
According to the present invention, the foregoing object is attained by providing a color-image processing apparatus for outputting signals for a color image after applying a color correction to an original image input thereto, comprising of a generating means for generating color information of the plurality of color component signals, and arithmetic means for creating a frequency distribution based upon the color information from the generating means, wherein a color image is formed upon deciding processing coefficients of the color correction by an arithmetic function using the plurality of color components depending upon the distribution state of the frequency distribution.
Another object of the present invention is to improve an image processing method based upon a histogram of an original.
Furthermore, an object of the present invention is to economize the memory.
Another object of the present invention is to realize high-speed image processing.
Another object of the present invention is to provide a copying machine which processes an image with high quality.
Still another object of the present invention is to optimize the determination of coefficients.
Another object of the present invention is to optimize a LOG conversion.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.